The present invention relates to an optical functional element which can be used as an optical delay element, wavelength converter, optical switch, optical modulator, optical amplifier, optical memory, dispersion compensator, soliton generator, and the like.
In optical communications and optical circuits, improvements of various optical functional elements have been demanded. For example, optical communications require an optical switching that has an optical delay element to set a communication route. Since the optical switching cannot simultaneously process a plurality of optical signals, the optical delay element delays some of the optical signals to sequentially input them to the optical exchange or to process them by changing the order of the signals. Conventionally, as an optical delay element, an optical fiber which has a length corresponding to a required delay time and is arranged in addition to the optical fibers used as signal lines is used, and the optical signal to be delayed is input to and output from this optical fiber via optical switches. However, the optical fiber used as the optical delay element must have a length of several ten meters, and a large space is required to accommodate the optical fiber.
In a wavelength converter (second harmonic generation, SHG) that uses the second-order nonlinear optical effect, it is important to realize phase matching between the fundamental wave and second harmonic wave (SH wave) in terms of the wavelength conversion efficiency. However, the refractive index of a substance depends on the wavelength of light, and it is hard to realize phase matching. As a conventional method of realizing phase matching, an angle tuning method that utilizes birefringence of a single crystal, Cerenkov-radiation type phase matching and quasi-phase matching are known. In consideration of the conversion efficiency, the method using the single crystal is advantageous. However, it is difficult to grow a large single crystal. Also, although angle tuning is realized in bulk, since in a waveguide the crystal orientation cannot be finely adjusted three-dimensionally, it is hard to realize angle tuning. Cerenkov radiation assumes strict angle of phase matching between the SH and fundamental waves and it can provide only a low conversion efficiency. In quasi-phase matching, it is difficult to manufacture a complicated element structure. Also, since quasi-phase matching is attained by adding an offset to the shifted phase, phase matching is not achieved in the strict sense.
Due to these problems, it is difficult to put the wavelength converter into practical applications although various applications such as a light source of an optical disk are expected. In particular, an organic nonlinear optical material is advantageous as compared to an inorganic material since it has a very large nonlinear susceptibility, but has not reached the stage of practical use due to problems in the manufacturing process of elements. Also, when a polymeric second-order nonlinear material or a material prepared by dispersing an organic nonlinear optical material in a polymer is used, an orientation treatment by means of poling is required, and a measure against orientation relaxation must also be taken.
In an element such as an optical switch that uses the third-order nonlinear optical properties, the nonlinear susceptibility is known to be larger as the phase relaxation time becomes longer. For this reason, the nonlinear susceptibility becomes large in the resonance region of absorption, and becomes small in the nonresonance region. However, since signal light is absorbed in the resonance region, the intensity of the switched light decreases, or decomposition of the functional material occurs due to heat generated by absorption. In consideration of this fact, the third-order nonlinear optical element that operates in the resonance region is not preferred in practical applications. In view of such problems, elements that operate in the non-resonance region have been demanded. However, since such elements have a small nonlinear susceptibility, as described above, they have not reached the level of practical use yet.
As the methods of optical modulation, electro-optic modulation (EO modulation) using the Pockels effect, absorption modulation that attains modulations based on an absorption change by applying an electric field and direct modulation that controls the injection current of a semiconductor laser are known. The direct modulation does not require any special modulator and requires only a small total number of parts. However, it is hard to achieve high-speed modulation, and the half width of wavelength undesirably broadens. In view of such drawback, high-speed external modulators using materials such as GaAlAs and, LiNB.sub.3 have been developed, but satisfactorily high performance has not been obtained to date. For this reason, a modulator which can be easily manufactured, allows high-speed modulation, and has a high ON-OFF ratio, has been demanded.